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Is terminal velocity constant?

  1. Dec 18, 2005 #1
    Im fairly ignorant on the theory of terminal velocity.

    It says that matter has a maximum speed in a certain direction.

    Is it possible that another factor could be the distance to the centre of gravity, which ever way it may be attracted?

    If a marble is dropped at 999m (just work with the figures plz) takes about 200m to reach 'terminal velocity', could this certain speed be relative to its distance. Mabye it can't increase in speed for that distance.

    Its true the closer to the centre of gravity the greater effect it has on an object.

    couldnt once this object reach's its 'relative terminal velocity' it could then increase at a steady pace till the point where the centre of this gravity is completely still?

    You could say that it is possible that it is implieng that the pull of gravity is its maximum but that is saying:

    a) terminal velocity can never be reached

    b) is only possible in a vacuum.

    Plz help im lost. thnx :confused:
  2. jcsd
  3. Dec 18, 2005 #2
    If you think of an object, say your marble, falling under the influence of gravity in the atmosphere there are two forces acting on it. First, there's the force of gravity. Second, there's the force exerted on the marble by all the air molecules it's running into. The faster the marble goes, the more molecules it hits and the larger the force is. When the magnitude of the force from the air equals the magnitude of the gravitational force the marble will no longer accelerate, that velocity is called the terminal velocity. It varies according to the mass and aerodynamic properties of the object you're considering.
  4. Dec 18, 2005 #3


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    Staff: Mentor

    It does also vary according to altitude because of the density of the air (drag) and pull of gravity both varying by altitude.
  5. Dec 18, 2005 #4


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    You are omitting 2 important ingredients: a retarding medium which resists the motion with a force that increases with speed, and that the motive force must be constant.
    a) is true, but hardly relevant. the deviation of velocity from terminal decreases exponentially in most cases, so quickly becomes negligible.
    b) is not true. As I stated, without a retarding medium to provide resistance, terminal velocity is infinite.
    Last edited: Dec 18, 2005
  6. Dec 19, 2005 #5
    I wonder if the OP isn't confusing different notions, the idea that everyday objects in free fall have a terminal velocity (atmosphere being assummed) and the fact that there is an absolute speed limit in the universe, the speed of light (in a vacuum).

    Terminal velocity as it applies to (say) a person jumping out of an airplane is all about the friction between the person and the air. There are a few presumptions that go into this scenario. The person is in free fall, meaning that there are no forces (like a rocket motor) pushing him down and no forces (like an updraft) pushing him up. The air density is considered constant, or in a more complicated model is a function of altitude. The frictional coefficient between the person and the air is constant, and the surface area of the person is constant (these last two factors are changed when the person opens his parachute, which presumably he has with him).

    The gravitational force pulling the man down is essentially linear, so in the absence of friction his velocity increases at a constant rate. This is the definition of a constant accelleration. But friction with the air varies as the square of the velocity, so the faster he goes the harder it is for the pull of gravity to make him go faster. Once the person reaches the speed at which the frictional force equals the gravitational force, accelleration goes to zero and the person falls with a constant velocity. This is the terminal velocity.

    For different objects with different coefficients of friction and surface areas (consider a feather and a bowling ball, or even a bowling ball covered with feathers, or a person with and without a parachute), each has it's own terminal velocity for a given force of gravity and a given atmosphere condition. The force of gravity doesn't change much over distance, unless one is very close to the center of mass of a very dense massive body. If that's the case, terminal velocity is probably the least of their worries.

    The speed of light limit is something else entirely, and this is an inherent limit on any type of matter, and has nothing to do with friction. I don't know if that was confusing you or not, but your general use of the term "matter" made me suspect the possibility.
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